soundproofing a wall, part 1

Today, I bring you a post outlining how to start the process of soundproofing a wall.

I got a lot of help and advice on this part from the good folks at Super Soundproofing here in San Diego — I’m trying to do my best to follow their direction, but I may be missing something here and there. With that said, though, here’s how I’m doing soundproofing for the first wall between two bedrooms.

On this wall, I only pulled the drywall off of one side, so it’s not getting the full double-sided soundproofing treatment that the living room to bedroom wall is getting. But for sound transmission between two bedrooms, I think this’ll do OK.

The steps to complete the wall are basically:

Seal all holes and butter any boxes with acoustical caulk

Install cotton insulation batts between studs

Lay and attach mass-loaded vinyl across the studs

Seal mass-loaded vinyl edges with acoustic caulk, and butt joints with lead tape

I used an SPL meter to measure the difference sound level after adding the various pieces, with the stereo in the next room turned to 25, playing The Puppini Sisters’ “Mister Sandman”. (Not quite pink noise, but it was something fun to listen to.)

So here are the steps I followed:

First, any wall penetration on the other side needs to be sealed with acoustical caulk that dries very flexible and rubbery (and stinky for the first week), to prevent sound transmission from air gaps. As well, any box (like the electrical boxes for power and ethernet/coax) needs to be buttered up with the stuff in order to reduce vibrations and resonances of the box.

The electrical boxes:

The ethernet boxes:

And the heating vent into the other room:

At this point, the sound from the next room was pretty loud:

Next step is to add insulation made from cotton denim. I’m not 100% certain that it’s made from recycled blue jeans, but it sure looks like it. These go in just like fiberglass insulation — just shove it into the stud cavities without compressing it — except there’s no itching. The bad part is that it costs between 2 and 4 times as much as fiberglass, so we’re not going to be using it for the outside walls.

You have to make slits for cables, just like normal fiberglass:

You also have to cut it (actually it’s more like rip it) to fit narrower cavities:

This was actually a major help for reducing the sound. It’s still loud, mind you, but certainly muffled after the blue jeans:

86.3 to 74.3 dB is a reduction of 12dB, which (since dB is a log scale) is a little over half of the original sound transmission.

Next, unroll the mass loaded vinyl. I used 1/8″ thick, which is the thinnest you can get — and weighs in at 100 pounds per 25 foot roll. Heavy stuff. To attach it to the studs, you use nails with plastic washers attached to them — they may be called “tin caps”, or in my case, “plastic round cap roofing nails, 1 inch”. I tried to keep the nails in a line that was more or less even, since drywall screws are going to have to get screwed in and I won’t be able to spot the nails visually, and the drywall screws won’t be able to be screwed in through the plastic caps.

You don’t want to stretch it tight on the wall — it should be even, but somewhat loose and able to flap back and forth a bit. This will help absorb some of the sound.

Here’s the first pass:

Next time — the second pass, sealing this layer, and more measurements!

You’re right — I remembered the ‘half’ from somewhere, and didn’t take time to do the math. But yeah, for just smearing around some goop and throwing a few pieces of material in the wall, that’s a decent reduction.

And yes, the meter’s mine. I got it a while back and also use it for adjusting equalizers wherever; it even helps a little with a crappy car radio that only has bass/treble and ‘xtreme bass boost’. The display is its “competition car audio” mode, for when the goal is to make your system as ear-bleedingly loud as possible.

Working on part 2; I’m waiting on a few other things and a drywall delivery, then there’ll be more.

I have confirmed that the stuff is, in fact, shredded blue jeans. I wonder what the R value is. I need to finish my ham radio shack and it’d be nice to have insulation I could stuff as the studs are a non-standard distance apart. I also have to figure out how to run the electricity and other things.

Our av guy also told me about what he did for his dad’s recording studio; 1/2 thick plywood on either side and sand between the walls. Apparently took a full truck.